1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer to form an image by pixels and, more particularly, to an image forming apparatus in which a plurality of resolutions are selectively switched and an image can be printed.
2. Related Background Art
Hitherto, for instance, a laser beam printer has been known as such a kind of apparatus. A printer engine unit of the apparatus has a construction as shown in FIG. 1.
In FIG. 1, reference numeral 1 denotes a photo sensitive drum as an electrostatic latent image carrier; 2 indicates a semiconductor laser; 3 a developing device for depositing a toner to an electrostatic latent image and for visualizing; 4 a rotary polygon mirror to scan a beam which is emitted from the semiconductor laser 2 onto the photo sensitive drum 1; 5 a charger; 6 a transfer charger to copy transfer a toner image onto a print paper 10; 7 a fixing device to fix the toner which was transferred onto the print paper 10; and 8 resist rollers for correcting the oblique movement of the paper which was fed by a paper feed roller 9 by once abutting the paper to the rollers and for adjusting the front edge of the page and the image print start timing of the laser. Reference numeral 12 denotes a cleaner to clean the surface of the photo sensitive drum 1 and 14 indicates a scanner motor to drive the polygon mirror 4.
To switch a resolution in the printer having such a construction, for instance, it is necessary to switch a rotational speed of the scanner motor 14 and to switch a period of image clock corresponding to one dot of the turn-on/off of the semiconductor laser 2. In the conventional switching method, a switch or the like provided for the printer is detected at the time of the turn-on of the power source and the resolution is switched when the printer is made operative.
Therefore, the resolution cannot be switched in a state of the power ON.
On the other hand, FIG. 2 shows a system in which the resolution is switched by a resolution changeover request command which is sent from a controller 30 as a generation source of image information. In FIG. 2, reference numeral 20 denotes a command which is sent from the controller 30 to a printer engine 34 and 21 indicates a status which is contrarily sent from the printer engine 34 to the controller 30. The command and status signals are communicated asynchronously with an image signal. Reference numeral 23 denotes a timing signal of the image signal. A horizontal sync signal (well-known beam detection signal) and a vertical sync request signal are sent from the printer engine 34 to the controller 30. The vertical sync signal is sent from the controller 30 to the printer engine 34. When the controller 30 receives the vertical sync request signal from the printer engine 34, the controller 30 sends a vertical sync signal to the printer engine 34 and, thereafter, sends image data 26 of one line synchronously with a horizontal sync signal from the printer engine 34 and with an image clock 27.
On the basis of the image data 26, the semiconductor laser 2 in an optical system unit 33 of the printer engine 34 is turned on and off. A laser beam emitted from the laser 2 scans on the photo sensitive drum 1 by the polygon mirror, thereby forming an image.
In addition to the optical system unit 33, the printer engine 34 also includes: a scanner motor driver 32 to drive the scanner motor; and a CPU 31 to drive and control the photo sensitive drum 1, scanner motor 14, and the like.
When the controller 30 outputs a resolution changeover request command as a command 20, the printer engine 34 finishes the delivery of the paper which is at present being printed and changes a rotational speed of the scanner motor 14 to a predetermined value and, thereafter, restarts the paper feeding operation.
However, in the conventional example, in the case of switching the resolution by the detection of the switch at the time of the turn-on of the power source, the resolution corresponding to the switch is set simultaneously with the start of the operation of the printer. Therefore, for instance, the resolution cannot be switched every page and the like. The requests from the controller 30 cannot be sufficiently satisfied.
On the other hand, even in a printer which can switch the resolution by the command 20 from the controller 30, after the paper 10 which is at present being printed had been once delivered and the rotational speed of the scanner was changed, the paper feeding operation is started. Thus, a throughput largely deteriorates due to the switching of the resolution.
Further, the above conventional technique has a drawback such that when the print paper 10 has once been fed, the resolution for the paper 10 cannot be switched, so that it is difficult for the controller 30 to process such a situation.
In the above image forming apparatus, a predetermined amount of laser beam is emitted in order to obtain the foregoing beam detection signal (hereinafter, referred to as a "BD signal") as a horizontal sync signal serving as a reference of the image forming position in the main scanning direction.
However, due to the light emission to detect the BD signal, for instance, an image of about two lines is formed as an electrostatic latent image into an image effective area on the photo sensitive drum 1. Such an unnecessary electrostatic latent image which is not an inherent copy transfer image is also developed as a toner image and the toner is transferred from the drum 1 to the transfer charger 6.
That is, hitherto, when the rotational speed of the scanner motor 14 is changed in order to switch the resolution, an unnecessary toner image is certainly formed when the BD signal is detected.
In the case of the system such that the transfer charger 6 is used upon transfer, the toner drops to a region around the transfer charger 6 and becomes a cause of a failure of the transfer charger 6 or a deterioration of the performance thereof.
In the system in which a transfer roller (not shown) is used in place of the transfer charger 6, the toner image is directly transferred to the transfer roller.
Such a fouling of the toner onto the transfer roller causes an inconvenience such that even if an amount of toner which is deposited onto the transfer roller is very small, what is called a "back surface dirt" of a recording medium occurs by the elapse of time.
On the other hand, in a recording apparatus which can print images to both surfaces of a paper, such a "back surface dirt" becomes a dirt of the recording surface and causes an inconvenience such that the recording picture quality is remarkably deteriorated.